[#9] Tests — fixed pairs

[javaisbetterthanpython]

Summary

• Add a dedicated describe("fixed pairs") block in test/heuristics.spec.tsx with shared assertion helpers
• Cover all four acceptance criteria: same-team over 10 rounds, two fixed pairs with unpaired players, atomic sit-outs, and mid-game regenerate
• Retain volunteer sit-out test for fixed-pair partner pull-through

Test plan

• yarn test:ci passes (18 tests)
• yarn build passes
• yarn lint passes
• Fixed pair always teammates across 10 generated rounds
• Two fixed pairs + unpaired players produce valid matches
• Fixed pair sits out together when sit-outs required
• Adding fixed pair mid-game keeps pair together on regenerate

Closes #9

Made with Cursor

[javaisbetterthanpython]

APPROVE

Focused test-only change for #9. Refactors fixed-pair coverage into a dedicated describe("fixed pairs") block with shared assertFixedPairOnSameTeam and assertValidRound helpers. All four acceptance criteria are covered with clear test names and assertions:

• 10 rounds, same team — loops 10 getNextRound calls with history and asserts pair a/b stay teammates or sit out together each round.
• Two fixed pairs + unpaired — 10 players, 2 courts, 5 rounds; validates round structure/partitioning and both fixed pairs.
• Atomic sit-outs — 6 players / 1 court forces 2 sit-outs; asserts length and paired sit-out behavior across 10 rounds.
• Mid-game regenerate — 5 rounds without fixed pairs, then regenerates the last round with [["a","b"]] and asserts they stay together.

Retains the volunteer sit-out pull-through test. Scope is limited to test/heuristics.spec.tsx. Local CI: test:ci (18 passed), build, and lint all green (pre-existing hook warnings only). GitHub CI also passing.